1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
2. Description of the Related Art
The following document discloses a technology relative to a conventional semiconductor laser with electro-absorption type modulator.
Document: H. Yamazaki, M. Yamaguchi, Y. Sakata, Y. Inomoto, K. Komatsu; Opto-Electronics Res.Labs., ULSI Dev.Labs., NEC Corp. xe2x80x9cAn investigation on simultaneously demonstrated low voltage and high power operation in DFB-LD/modulator integrated light sourcesxe2x80x9d THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS; TECHNICAL REPORT OF IEICE. LQE95-18 (1995-06)
The semiconductor laser disclosed in the above document has features of having a window area in order to reduce an edge (end) reflectance. With reference to FIG. 28 to FIG. 31, a manufacture process of conventional semiconductor laser will be described below.
A grating 3 is formed in a laser forming region LR of a substrate (InP) 1, and thereafter, a mask pair 5 is formed with respect to the substrate 1. A mask width of each mask constituting the mask pair 5 differs in the laser forming region LR and a modulator forming region MR. The mask width in the laser forming region LR is wider than that in the modulator forming region MR. For example, the narrow mask width is adjusted to 5 xcexcm; on the other hand, the wide mask width is adjusted to 50 xcexcm. Moreover, an interval between masks constituting the mask pair 5 is adjusted to 1 to 3 xcexcm.
A material InGaAsP is selectively grown using a metal organic vapor phase epiytaxy (MOVPE) method. As a result, an active layer (multiple quantum wells (MQW) structure) 7 is formed in the laser forming region LR; on the other hand, an absorptive layer 9 is formed in the modulator forming region MR. The material InP is grown with respect to these active layer 7 and absorptive layer 9 so that a cladding layer 11 is formed (see FIG. 28).
Thereafter, the mask pair 5 is removed so that a mask (SiO2) 13 is formed. Then, using the mask 13 thus formed, part of the cladding layer 11 and the absorptive layer 9 is etched so that a window region WR is formed (see FIG. 29).
Then, a mask pair 15 is formed, and InP is further grown on the cladding layer 11 so that a cladding layer 17 including the cladding layer 11 is formed. A contact layer 19 is formed on the cladding layer 17 thus formed (see FIG. 30).
After the mask pair 15 is removed, a metal material is evaporated on the surface of the contact layer 19 so as to form a predetermined pattern. Moreover, a metal material is vapor-deposited onto the back side of the substrate 1. After the evaporation, annealing treatment is subject to these materials so that the vapor-deposited metal is alloyed. By doing so, a laser p-side electrode 21 is formed in the laser forming region LR; on the other hand, a modulator p-side electrode 23 is formed in the modulator forming region MR, and further, an n-side electrode 25 is formed onto the back side of the substrate 1. In this case, the contact layer 19 between the laser p-side electrode 21 and the modulator p-side electrode 23 is removed before evaporating the metal material.
The end portion of semiconductor device is cloven and made into a chip, and thereafter, an end face of the modulator forming region MR is coated with a low reflection film 27. In a chipped semiconductor laser with modulator, an optical axis length of the laser forming region LR is set to a range from 300 to 7001 xcexcm, and an optical axis length of the modulator forming region MR is set to a range from 50 to 250 xcexcm, and further, an optical axis length of the window region WR is set to a range from 10 to 50 xcexcm (see FIG. 31).
In the semiconductor laser with modulator, when a modulation voltage is applied to a modulator during laser oscillation, the modulator is operated so as to absorb a laser beam. In this case, when a light absorption exceeds a predetermined value, an element breakdown occurs in an interface between the modulator and laser. However, the conventional semiconductor laser with modulator has no structure effective for preventing the element breakdown phenomenon as described above. For this reason, in the conventional semiconductor laser with modulator, in the case where a laser output is slightly enhanced, the light absorption of modulator exceeds a limit value; as a result, a problem has arisen such that an element breakdown occurs.
The present invention has been made in view of the above problem in the prior art. It is, therefore, an object of the present invention to provide a semiconductor device, which includes a first region having an optical waveguide layer, and a second region having a light receiving layer receiving a light from the first region, and can improve a light intensity of the light receiving layer, and to provide a manufacturing method thereof.
In order to solve the above problem and to achieve the above object, according to a first aspect, the present invention provides a semiconductor device, which includes a first region having an optical waveguide, and a second region having a light receiving layer receiving a light from the first region. The semiconductor device includes an optical confinement layer, which is formed on the optical waveguide layer and the light receiving layer and has a shape extending to an optical axis direction. Further, a width of contact surface of the optical confinement layer with the light receiving layer is wider than a width of contact surface of the optical confinement layer with the optical waveguide layer.
With the above construction of the present invention, when the light receiving layer receives a light propagated through the optical waveguide layer, it is possible to prevent an element breakdown, which is likely to occur in a junction interface between the optical waveguide layer and the light receiving layer.
According to a second aspect, the present invention provides the semiconductor device, which further includes a coupling part for reducing a density of light propagated through the optical waveguide layer, between the first and second regions. Preferably, the coupling part has a direction propagating a light propagated through the optical waveguide layer other than the optical axis direction of the optical waveguide layer and the light receiving layer.
With the above construction of the present invention, even if an intensity of light propagated through the optical waveguide layer is high, it is possible to moderate a damage given to the light receiving layer when receiving a light.
According to a third aspect, the present invention provides a manufacturing method of a semiconductor device including a first region having an optical waveguide layer, and a second region having a light receiving layer receiving a light from the first region. The manufacturing method comprises the following steps of: forming the optical waveguide layer and the light receiving layer; growing the optical confinement layer on the optical waveguide layer and the light receiving layer so that the optical confinement layer in a selective region selected from the first region is formed thicker than the optical confinement layer in other regions; and etching the optical confinement layer so that the optical confinement layer is formed into a shape of ridge extending to an optical axis direction, and a shape of inverse mesa having a width formed so as to gradually becomes narrow in its depth direction.
With the above manufacturing method of the present invention, in the contact surface of the optical confinement layer formed into an inverse-mesa ridge shape by etching with the optical waveguide layer and the light receiving layer, the following relation is formed. More specifically, a width of contact surface of the optical confinement layer with the light receiving layer is wider than a width of contact surface of the optical confinement layer with the optical waveguide layer. Therefore, in the semiconductor device manufactured by the above manufacturing method, when the light receiving layer receives a light propagated through the optical waveguide layer, it is possible to prevent an element breakdown, which is likely to occur in a junction interface between the optical waveguide layer and the light receiving layer.
Preferably, the selective region is a region held between a mask pair formed in the first region. Further, the optical confinement layer is selectively grown with respect to other regions in the region held between the mask pair. Therefore, according to the manufacturing method, the optical confinement layer in the selective region is formed thicker than the optical confinement layer in other regions.
According to a fourth aspect, the present invention provides a manufacturing method of a semiconductor device including a first region having an optical waveguide layer, and a second region having a light receiving layer receiving a light from the first region. The manufacturing method comprises the following steps of: forming the optical waveguide layer and the light receiving layer; forming the optical confinement layer on the optical waveguide layer and the light receiving layer; and etching the optical confinement layer using a mask, which extends from the first region to the second region and has a mask width in the second region wider than a mask width in the first region.
With the above manufacturing method of the present invention, a width of contact surface of the etched optical confinement layer with the light receiving layer is wider than a width of contact surface of the optical confinement layer with the optical waveguide layer. Therefore, in the semiconductor device manufactured by the above manufacturing method, when the light receiving layer receives a light propagated through the optical waveguide layer, it is possible to prevent an element breakdown, which is likely to occur in a junction interface between the optical waveguide layer and the light receiving layer.
For example, the present invention is applicable to a semiconductor laser with modulator used as semiconductor device. In this case, the first region is equivalent to a region where a laser device is formed, and the second region is equivalent to a region where a modulator for modulating a laser beam outputted by the laser device is formed. Further, the optical waveguide layer is equivalent to an active layer, the light receiving layer is equivalent to an absorptive layer, and the optical confinement layer is equivalent to a cladding layer.